Mgsc4 align

[jensen-yan]

Summary by CodeRabbit

• Bug Fixes

  • Resolved branch prediction gate logic issues to prevent negative threshold values from disabling confidence gates and negatively impacting prediction accuracy.

• Configuration

  • Enhanced configuration options for branch predictors in decoupled processor system architectures.

• Performance

  • Improved precision of branch prediction threshold counters by increasing their width for more accurate tracking and calculations.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Enabled MGSC for BTB path in KMHV3 configuration, increased MGSC update threshold counter width from 12 to 14 bits, and added clamping logic to prevent negative threshold values from affecting prediction gates during single prediction generation and global threshold updates.

Changes

Cohort / File(s)	Summary
Configuration Updates configs/example/kmhv3.py	Enabled mgsc.enabled flag for DecoupledBPUWithBTB configuration from False to True
Parameter Width Adjustment src/cpu/pred/BranchPredictor.py	Increased updateThresholdWidth parameter in BTBMGSC class from 12 to 14 bits
Threshold Validation Logic src/cpu/pred/btb/btb_mgsc.cc	Added clamping to ensure total_thres remains non-negative in generateSinglePrediction; corrected negative updateThreshold values to zero in updateGlobalThreshold

Estimated Code Review Effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly Related PRs

• Add kmhv3 #609 — Modifies KMHV3 branch-predictor configuration and makes MGSC-related changes in predictor code
• Resolve align #621 — Modifies the same mgsc.enabled flag in kmhv3.py for DecoupledBPUWithBTB (takes opposite approach)
• Test align #612 — Updates kmhv3.py branch-predictor configuration with MGSC enablement under KMHV3

Suggested Reviewers

• tastynoob
• Yakkhini

Poem

🐰 In thresholds once too loose we'd roam,
Now clamped to zero, safe at home,
14 bits now guide the way,
MGSC gates shine bright today! ✨

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Title check	❓ Inconclusive	The title 'Mgsc4 align' is vague and does not clearly describe the specific changes made in the pull request, which involve enabling MGSC and adjusting threshold parameters across multiple files.	Consider using a more descriptive title that summarizes the main changes, such as 'Enable MGSC and adjust threshold parameters for branch prediction' or 'Update MGSC configuration and threshold clamping logic'.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

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[github-actions]

🚀 Coremark Smoke Test Results

Branch	IPC	Change
Base (xs-dev)	2.1727	-
This PR	2.2186	📈 +0.0459 (+2.11%)

✅ Difftest smoke test passed!

[XiangShanRobot]

[Generated by GEM5 Performance Robot]
commit: 34f1ac8
workflow: gem5 Align BTB Performance Test(0.3c)

Align BTB Performance

Overall Score

	PR	Master	Diff(%)
Score	17.88	17.56	+1.85 🟢

[Generated by GEM5 Performance Robot]
commit: 34f1ac8
workflow: gem5 Align BTB Performance Test(0.3c)

Align BTB Performance

Overall Score

	PR	Previous Commit	Diff(%)
Score	17.88	17.88	+0.03 🟢